4. The Sm)Nd method

Sm is a rare earth element with seven naturally occurring isotopes. Of these ¹⁴⁷Sm, ¹⁴⁸Sm and ¹⁴⁹Sm are all radioactive, but the latter two have such long half-lives (ca. 10¹⁶ yr) that they are not capable of producing measurable variations in the daughter isotopes of ¹⁴⁴Nd and ¹⁴⁵Nd, even over cosmological intervals (10¹⁰ yr). However the half-life of ¹⁴⁷Sm (106 Byr) is sufficiently short to produce small but measurable differences in ¹⁴³Nd abundance over periods of several million years, thus providing the basis for the Sm)Nd dating method. This half-life, equivalent to a decay constant of 6.54 H 10^!12 yr^!1, is the weighted mean of several determinations, and yields ages consistent with U)Pb dating (Lugmair and Marti, 1978).

Another samarium isotope, ¹⁴⁶Sm, is not naturally occurring, but has a relatively long half-life of 103 Myr. If Sm/Nd fractionation occurred within a few hundred million years of Sm nucleosynthesis then variations in the abundance of the daughter product, ¹⁴²Nd, might be observable in different terrestrial reservoirs. This subject will be discussed under the heading of ‘extinct nuclides’ (section 15.5.5).

4.1 Sm)Nd isochrons

Considering a given system, such as an igneous rock or mineral, we can write the following equation based on the decay of ¹⁴⁷Sm:

¹⁴³Nd = ¹⁴³Nd_I + ¹⁴⁷Sm (e^8t ! 1) [4.1]

where I signifies initial abundance and t is the age of the system. In view of the possibility of ¹⁴²Nd variation (due to ¹⁴⁶Sm), it is convenient to divide through by ¹⁴⁴Nd, the second-most abundant isotope of Nd. Thus we obtain:

¹⁴³Nd (¹⁴³Nd) ¹⁴⁷Sm

)))) = ())))) + )))) (e^8t ! 1) [4.2]

¹⁴⁴Nd (¹⁴⁴Nd)_I ¹⁴⁴Nd

This equation has the same form as that for Rb)Sr (section 3.2) and can be plotted as an isochron diagram. However, because Sm and Nd have very similar chemical properties (unlike Rb and Sr), large ranges of Sm/Nd in whole-rock systems are rare, and in particular, low Sm/Nd ratios near the y axis are very rare. Therefore, because of the difficulty of obtaining a wide range of Sm/Nd ratios from a single rock body, and because of the greater technical demands of Nd isotope analysis, the Sm)Nd isochron method was generally applied to problems where Rb)Sr isochrons had proved unsatisfactory. Many of these applications were also made before the U–Pb zircon method had reached its present level of development (section 5.2.2). Therefore some of these units have subsequently been dated to greater accuracy and precision by the U–Pb method. However, it is important to review a few case studies to show the development of the method.

4.1.1 Meteorites

Chondritic meteorites have been readily dated by the Rb)Sr method, but achondrites are more problematical. Bulk samples usually have low Rb/Sr ratios, yielding ages of low precision, while separated minerals in many achondrites yield Rb)Sr ages below 4.5 Byr, indicative of disturbance. The Sm)Nd system in separated minerals from achondrites is more resistant to re-setting, yielding better age estimates. The first Sm)Nd dating study was performed by Notsu et al. (1973) on the achondrite Juvinas, but with low analytical precision. Lugmair et al. (1975) obtained much more precise results on minerals from the same meteorite (Fig. 4.1) yielding an age of 4560 " 80 Myr (2F).

Numerous other basaltic achondrites have been dated by Sm–Nd, and with the exception of Stannern (Lugmair and Scheinin, 1975), all yield ages in the range 4550 ) 4600 Myr. These age determinations have since been superseded by Pb–Pb dating studies (section 5.3). However, the good agreement between the Sm–Nd and Pb–Pb dates has served the important function of confirming the ¹⁴⁷Sm half-life of 106 Byr.

Fig. 4.1. Sm)Nd isochron for whole-rocks and minerals from the basaltic achondrite Juvinas. Nd isotope ratios are affected by the choice of normalising factor for mass fractionation. Data from Lugmair et al. (1975).

Sm)Nd dating of chondritic meteorites was not a high priority, due to the success of other methods. However, the isotopic composition of the chondrites is a critical benchmark for the evolution of solar system bodies such as the Earth, because chondrites are believed to represent the nearest approach to the primordial solar nebula. DePaolo and Wasserburg (1976a) coined the acronym CHUR (chondritic uniform reservoir) for this benchmark, but in the absence of isotopic data for chondrites had to use Lugmair’s (1975) ¹⁴³Nd/¹⁴⁴Nd ratio of 0.511836 from the achondrite Juvinas as an indicator of the present day CHUR value (using a fractionation normalisation to ¹⁴⁶Nd/¹⁴²Nd = 0.636151 for Nd analysis as the oxide).

This value was tested by direct Sm)Nd analysis of chondrites by Jacobsen and Wasserburg (1980). They obtained a whole-rock isochron with an age of ca. 4.6 Byr, but more importantly, the measured ¹⁴³Nd/¹⁴⁴Nd ratios clustered closely around the original Juvinas measurment (dashed line in Fig. 4.2). The intersection of this value with the isochron regression led to a ¹⁴⁷Sm/¹⁴⁴Nd ratio of 0.1967 for CHUR. Jacobsen and Wasserburg compared this value to the average of 64 elemental Sm/Nd analyses of chondrites (Fig. 4.3), and demonstrated good agreement between the two values.

Fig. 4.2. Sm)Nd isochron diagram for whole-rock samples of six different chondrites. SS = St Severin; MU = Murchison; GU = Guarena; PR = Peace River; ALL = Allende. JUV = new analysis of the Juvinas achondrite. The large apparent errors are due to very expanded axis scales. After Jacobsen and Wasserburg (1980).

In 1981, Wasserburg et al. revised the isotopic composition of their oxide correction and modified their recommended ¹⁴³Nd/¹⁴⁴Nd value of CHUR to 0.511847. However, most workers use the alternative normalisation convention (to ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219) which was proposed by O’Nions et al. (1977) for Nd analysis as the metal (section 2.2.2). This leads to the corresponding present day CHUR values: ¹⁴³Nd/¹⁴⁴Nd = 0.512638 and ¹⁴⁷Sm/¹⁴⁴Nd = 0.1966 (Hamilton et al., 1983).

Fig. 4.3. Histogram of chondritic ¹⁴⁷Sm/¹⁴⁴Nd ratios determined from elemental analysis, compared with the value from Fig. 4.2 (arrow). Ordinary chondrites are sub-divided into compositional classes (H, L, LL). After Jacobsen and Wasserburg (1980).

4.1.2 Low grade meta-igneous rocks

The long half-life of ¹⁴⁷Sm makes it most useful for dating in the Precambrian. Therefore, most early Sm)Nd work was focussed on the determination of crystallisation ages for Archean igneous rocks. In such suites the Rb)Sr or K)Ar methods had often shown open system behaviour, and precise U–Pb dates were not yet available. The Stillwater Complex (DePaolo and Wasserburg, 1979) provides a good example of such an application.

Rb)Sr data on three separated minerals from a single adcumulus unit of the Stillwater layered series form a scatter which does not define an isochron, (Fig. 4.4a). This was attributed to open-system behaviour of Rb-Sr in minerals. However, Sm)Nd data on the same samples defined an excellent linear array (Fig. 4.4b), from which DePaolo and Wasserburg calculated an age of 2701 " 8 Myr (2F). The analysis of separated minerals provided a much greater range of Sm/Nd ratios than whole-rock samples, but raised the possibility that Sm–Nd mineral systems might have been opened by the event that disturbed Rb–Sr systems. To test for this, DePaolo and Wasserburg also analysed six whole-rock samples from different levels in the intrusion with a wide range of plagioclase/pyroxene abundances. Sm)Nd data from these samples fell within analytical uncertainty of the mineral isochron (Fig. 4.4c), suggesting that the mineral isochron yields a true crystallisation age for the intrusion, and that the magma had a homogeneous initial Nd isotope composition.

Fig. 4.4. Isochron diagrams for the Stillwater Complex. a) Rb)Sr diagram showing scatter of mineral data; b) Sm)Nd mineral isochron; c) whole-rock data with reference line from (b). After DePaolo and Wasserburg (1979).

Subsequently, the Sm)Nd mineral age was corroborated by U)Pb dating of zircon from the chilled margin of the intrusion (Nunes, 1981), which gave an age of 2713 " 3 Myr (2F). However, Sm)Nd analysis of whole-rock samples from a wider stratigraphic range in the intrusion revealed larger variations of initial ratio (Lambert et al., 1989). This is not surprising, since the initial ratio of DePaolo and Wasserburg falls well away from estimated mantle values at 2.7 Byr, and is best explained by contamination of the magma by old crustal Nd from the Wyoming craton.

The Stillwater data emphasise the importance of combined mineral and whole-rock isochrons to verify the accuracy of Sm/Nd ages. However, this approach is not possible for fine-grained rocks such as Archean basalts and komatiites. In these situations, whole-rock analysis has often been used alone, but subtle changes to the slopes of whole-rock isochrons can be caused by analysing samples with slight variations in crustal contamination. A good example is provided by the Kambalda volcanics of western Australia.

McCulloch and Compston (1981) determined a composite Sm)Nd isochron on a suite of rocks comprising the ore-bearing Kambalda ultramafic unit, the footwall and hanging wall basalts, and an ‘associated’ sodic-granite and felsic porphyry. Although the whole suite yielded a good isochron age of 2790 " 30 Myr (Fig. 4.5), the basic and ultra-basic samples alone gave an older best-fit age of 2910 " 170 Myr.

Fig. 4.5. Composite acid)basic Sm)Nd isochron diagram for a suite of Archean rocks from Kambalda, Western Australia. Open symbols were omitted from the regression. After McCulloch and Compston (1981).

The danger of constructing a ‘composite’ Sm)Nd isochron of acid, basic and ultra-basic rocks which might not be co-magmatic was pointed out by Claoue-Long et al. (1984). These workers attempted to date the Kambalda lavas by the Sm)Nd method without utilising acid rocks. However, they were forced to combine analyses from komatiites and basalts in order to achieve a good spread of Sm/Nd ratios (Fig. 4.6). After the exclusion of one komatiite point from Kambalda and a suite of basalt lavas from Bluebush (40 km south of the main Kambalda sequence), ten data points gave an age of 3262 " 44 Myr (2F). This was interpreted as the time of eruption.

Fig. 4.6. Sm)Nd isochron diagram for whole-rock samples of Kambalda volcanics. ( € ) = komatiites; ( > ) = hanging-wall basalts; ( <> ) = Bluebush lavas; ( Î ) = ‘ocelli’ basalts; ( Ï ) = granites. Modified after Claoue-Long et al. (1984).

Chauvel et al. (1985) challenged this interpretation on the basis that Pb)Pb dating of the Kambalda volcanics and associated igneous sulphide mineralisation gave an age of 2726 " 34 Myr, which they argued to be resistant to re-setting by later events. They attributed the 3.2 Byr apparent Sm)Nd age to either variable crustal contamination of the magma suite by older basement, or possibly a heterogeneous mantle source. U)Pb dating of 3.4 Byr-old zircon xenocrysts in one of the hanging-wall basalts subsequently confirmed the contamination model (Compston et al., 1985).

In retrospect, danger signals can be seen in the whole-rock Sm)Nd data. Taken alone, the komatiites (including the sample rejected by Claoue-Long et al.) define a slope of less than 3.2 Byr, as do the Bluebush lavas (Fig. 4.6). Only the hanging-wall basalts define a slope of 3.2 Byr, but these are the samples which have probably suffered most contamination. Hence the data probably consist of a series of sub-parallel isochrons with ca. 2.7 Byr slope.

Similar effects have been demonstrated for komatiitic and basaltic lavas from Newton township in the Abitibi belt of Ontario. Cattell et al. (1984) obtained an apparent age of 2.83 Byr from a whole-rock Sm)Nd isochron of basic and ultra-basic lavas. However, a maximum eruption age of 2697 " 1 Myr was conclusively demonstrated by U)Pb zircon analysis of an underlying dacitic volcaniclastic rock. Cattell et al. plotted initial ¹⁴³Nd/¹⁴⁴Nd ratios at 2697 Myr against Sm/Nd (Fig. 4.7), yielding an erupted Sm)Nd isochron with an apparent age of 130 " 64 Myr (MSWD = 2.52). No age significance was attached to this pseudo-isochron, which was attributed to sampling of a variably depleted mantle source. However, contamination by older crustal rock is a strong possibility.

Fig. 4.7. Sm)Nd pseudo-isochron diagram for whole-rock samples of komatiite and basalt from Newton township, Ontario. The large apparent errors and scatter of data are due to a very expanded y-axis scale. After Cattell et al. (1984).

4.1.3 High grade metamorphic rocks

Most dating systems, including U–Pb zircon, can be re-set during high grade metamorphic events. However, the Sm)Nd method provides an opportunity to determine igneous protolith ages in high-grade metamorphic gneiss terranes where other systems are re-set. An example is provided by dating work on the Lewisian gneisses of NW Scotland. Whole-rock Rb)Sr, whole-rock Pb)Pb and U)Pb zircon ages on granulite-facies and amphibolite-facies gneisses are concordant at 2630 " 140, 2680 " 60 and 2660 " 20 Myr (2F) respectively (Moorbath et al., 1975; Chapman and Moorbath, 1977; Pidgeon and Bowes, 1972). However, these gneisses are generally very Rb- and U-depleted, suggesting that even large whole-rock samples were probably open systems for these elements during the depletion event.

A suite of whole-rock samples was dated by the Sm)Nd method (Hamilton et al., 1979) to see whether this system had remained undisturbed during the Badcallian metamorphic event which the other systems are presumed to date. An older age of 2920 " 50 Myr (2F) suggested that the gneisses had remained closed systems for Sm)Nd during granulite-facies metamorphism (Fig. 4.8). Hamilton et al. therefore interpreted the age as the time of protolith formation, which occurred 200 ) 300 Myr before the closing of U)Pb zircon and whole-rock Rb)Sr and Pb)Pb systems following metamorphism.

Fig. 4.8. Sm)Nd isochron for a mixed suite of granitic, tonalitic and layered basic gneisses from the Lewisian complex of NW Scotland, yielding an age of 2920 Myr. After Hamilton et al. (1979).

Despite the good quality of the Sm)Nd isochron, there are two problems with the sample selection. Firstly, the sample suite combined amphibolite- and granulite-facies gneisses, and secondly it contained a bimodal petrological suite, including tonalitic gneisses and basic rocks from the Drumbeg layered complex. Nevertheless, because the slope ages of the tonalites and mafic gneisses are very similar, the samples as a whole display good linearity, with an MSWD value of only 1.3 (using 1F errors of 0.1 % for Sm/Nd, and the individual within-run isotopic errors).

More detailed investigation by Whitehouse (1988) showed that the Drumbeg layered basic rocks retain a 2.91 Byr isochron age, but Sm)Nd whole-rock systems in intermediate to acid rocks have been re-set to the same age as the U)Pb zircon and other whole-rock systems. Ten samples of the latter suite define an errorchron with MSWD = 5.7, yielding an age (with estimate of geological error) of 2600 " 155 Myr (2F), shown in Fig. 4.9. Therefore, the isochron of Hamilton et al. (1979) apparently does correctly date the time of protolith formation, but only the basic rocks remained closed systems during the Badcallian event. This work shows that even whole-rock Sm–Nd isochrons can be perturbed by granulite facies metamorphism. However, it will be shown below that Sm)Nd model ages can preserve the approximate protolith ages of the intermediate gneisses, even though the isochron is disturbed (section 4.3.3). These model ages agree with the isochron age for the Drumbeg basic pluton.

Fig. 4.9. Sm)Nd ‘errorchron’ for Lewisian tonalitic gneisses, defining an age of 2600 Myr, attributed to granulite-facies metamorphism. After Whitehouse (1988).

4.1.4 High grade metamorphic minerals

Another area where the Sm)Nd isochron method has been widely applied is the dating of high grade metamorphic minerals. For example, garnet and clinopyroxene (cpx) have mirror-image distribution coefficients for rare-earth elements (REE), giving rise to a large range of Sm/Nd ratios, and hence allowing precise age determinations. The classic example of a garnet)cpx rock is eclogite, so this has been a major focus of Sm)Nd mineral dating. However, the relative immobility of the REE, which is such an asset in dating igneous crystallisation, is a problem in using the Sm)Nd method to date metamorphism. Mineral systems may be opened sufficiently to disrupt the original igneous chemistry, but not enough to completely overprint the system. An example is provided by the dating of Caledonian eclogites by Mork and Mearns (1986).

Some gabbros from western Norway were transformed to an eclogite mineralogy (garnet and omphacite), but retained a relict igneous texture. These samples did not reach isotopic equilibrium during Caledonian metamorphism. In contrast, nearby country-rocks which had been transformed to eclogite generated a mineral isochron with very low scatter (MSWD = 0.1) and a typical Caledonian metamorphic age of 400 " 16 Myr. The contrasting behaviour of the two eclogite types cannot be attributed to variable P,T conditions, since they are from within 1 km of each other. However, the country-rock eclogite had completely lost its pre-existing texture due to penetrative deformation and recrystallisation. Mork and Mearns suggested that such physical disruption might be necessary to achieve complete Nd isotopic equilibrium between metamorphic mineral phases.

Examination of the metagabbro Sm)Nd data at 400 Myr (Fig. 4.10) suggests that the main obstacle to isotopic homogenisation in this rock was the cpx phase. Because the transformation of augite to omphacite requires relatively minor cation exchange, complete re-setting of the Sm)Nd system in this mineral rarely occurs. In contrast, major chemical exchange and structural reorganisation are required to replace plagioclase with garnet, so complete re-setting is more likely. Hence, garnet ) whole-rock isochrons are more reliable than the garnet ) cpx pairs used in early dating work on eclogites (e.g. Griffin and Brueckner, 1980).

Fig. 4.10. Schematic illustration of the process of Sm)Nd remobilisation during the replacement of gabbro by an eclogite mineralogy. Modified after Mork and Mearns (1986).

Vance and O’Nions (1990) argued that garnet chronology provides a powerful tool for dating prograde metamorphism, in contrast to other methods, such as Ar)Ar and Rb)Sr, which date metamorphic cooling (section 10.5). Garnets are widely distributed in meta-pelitic rocks and develop in response to the changing P,T conditions of prograde metamorphism. Their chemistry (including the Sm)Nd system) is usually preserved during cooling because cation diffusion rates in garnet are very slow. The chemical composition of garnets can be used to calculate the P,T conditions of their growth, which, combined with age data, provide a method of determining progradational P,T - time paths for high grade metamorphic terranes. An application of this technique was demonstrated by Burton and O’Nions (1991) in a study of Caledonian regional metamorphism of a Proterozoic supracrustal sequence at Sulitjilma, northern Norway.

Burton and O’Nions dated garnet growth in adjacent graphite-bearing and graphite-free bands using the Sm)Nd and U)Pb isochron methods. An example is shown in Fig. 4.11 for a case where garnet rims and cores are distinct. The rims yield a slightly younger age, as would be expected. Note that the core is regressed with the whole-rock composition, whereas the rim is regressed with the matrix of the rock only, since this is the only part of the rock with which the rims were in diffusional contact at the time of their growth.

Fig. 4.11. Sm)Nd isochrons for whole-rock ) garnet-core, and matrix ) garnet-rim pairs from a graphite-free meta-pelite. Error bars indicate within-run precision. After Burton and O’Nions (1991).

Concordant results for the Sm)Nd and U)Pb techniques provide strong evidence that the ages for garnet ) matrix pairs are dating prograde mineral growth. When these ages are coupled with temperature data (Fig. 4.12) they indicate that garnet growth occurred first in the graphite-bearing assemblage, and subsequently at higher temperatures in the graphite-free assemblage. Peak metamorphic conditions were registered by the garnet rims of the latter assemblage. Hence, an average heating rate of 9 ^oC / Myr was calculated. On the other hand, Rb)Sr mineral ages on muscovite and biotite were used to deduce a cooling rate of 4 ^oC / Myr (Fig. 4.12).

Fig. 4.12. Temperature)time diagram for Sulitjilma supracrustals, northern Norway. Progradational heating rate is from garnet Sm)Nd ages ( Î ) and U)Pb ages ( # ). Retrogressive cooling rate is from Rb)Sr ages ( ! ). After Burton and O’Nions (1991).

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